Cellular telephone systems today transmit and receive both analog and digital signals. One example of an accepted analog system is the American Advanced Mobile Phone System (AMPS), which operates in the 824 to 894 MHz frequency band, and one example of an accepted digital system is the Groupe Speciale Mobile System (GSM), which operates in the 890 to 960 MHz frequency band. To be usable in both frequency bands, a base-station antenna must have wide operational bandwidth with a low VSWR across the entire bandwidth. For example, the AMPS and GSM frequency bands require a VSWR of less than 1.5 over a bandwidth of approximately 16%.
The base stations in a cellular telephone system typically use antennas mounted on 30-meter towers to communicate with mobile units over a range of up to 3 kilometers. Both sector-coverage and omni-directional-coverage antennas are employed, depending upon the cell geography and the traffic density. Sector-coverage antennas have traditionally been of the co-linear, corner-reflector type, can be physically large and are often considered objectionable from an environmental standpoint. As many as twelve corner-reflector antennas may be used on a single tower platform. These antennas often end up physically downtilted to minimize cell overshoot, and the resultant untidy appearance compounds the environmental problems.
Because of the need to communicate with mobile units having antennas installed vertically on vehicles, cellular systems typically use signals which are vertically polarized. It is, therefore, desirable to minimize the amount of energy transmitted with unwanted horizontal polarization ("cross-pol") because the transmission of such signals merely reduces the efficiency of the system.